1. Field of the Invention
The present invention relates generally to video display apparatus employing a display device having a fixed two-dimensional pixel arrangement such as a liquid crystal display (LCD) device and, more particularly, relates to video display apparatus applicable to a plurality of kinds of video signals having different formats.
2. Description of the Background Art
Conventionally, a television receiver has been proposed which is capable of dealing with any one of video signals from a plurality of kinds of television systems (formats) having different factors such as horizontal and vertical synchronizing frequencies, and such television receivers are disclosed, for example, in Japanese Patent Laying-Open No. 59-104866 and Japanese Patent Laying-Open No. 61-267469.
Firstly, as an example of such receivers, consider a video display apparatus having a deflection system such as a CRT display apparatus. Generally, in a video display apparatus having a deflection system, an image is displayed on the screen by scanning with an electron beam in an horizontal direction and a vertical direction using the deflection system. That is, the video signal displayed on the screen of CRT is a continuous signal in the horizontal direction while the video signal is a discontinuous signal sampled at an interval between the scanning lines in the vertical direction and sampled at a time interval corresponding to the vertical period in the time base direction as well.
A display region of an image formed on the screen as a two-dimensional space by such a video signal is determined by the distances scanned with the beam by the deflection system in the horizontal and vertical directions in the horizontal and vertical periods, that is, the deflection size, in addition to the number of scanning lines and the horizontal and vertical periods.
Accordingly, in a case of selectively displaying video signals of a plurality of formats having different horizontal and vertical synchronizing frequencies, that is, different numbers of scanning lines (in some cases, aspect ratios) in a display apparatus having a deflection system, the display apparatus may be adapted to be able to change the horizontal and vertical frequencies in the display apparatus according to the selected format.
In this case, the difference in the number of scanning lines for each format does not particularly cause any problem for display and the difference in the aspect ratio can be coped with by suitably setting the horizontal and vertical deflection sizes. If the aspect ratios are largely different from each other, processing such as time-base companding in the horizontal direction of the video signal is required in some degree in some cases.
Conversely, in a case of a video display apparatus without a defection system, employing a display device having the two-dimensional pixel arrangement fixed such as an LCD device, elements such as an interval between the scanning lines or the like are uniquely determined by the fixed pixel arrangement of the display device itself. Accordingly, in order to selectively display video signals of different formats on such a display device, a special counter-measure is required to solve the problem concerning the difference between the numbers of scanning lines or aspect ratios.
FIG. 1 is a block diagram showing one example of a video display apparatus for which such a counter-measure is taken. Referring to FIG. 1, the video display apparatus includes format converters 101a to 101n, a display processing unit 103, and a display unit 104 including a display device having a fixed two-dimensional pixel arrangement such as an LCD device. Signals of formats 1 to n having different horizontal and vertical frequencies are selectively provided to this video display apparatus.
Display processing unit 103 can only directly process a video signal of a specified format (in the figure, format 1) and cannot directly process signals of other formats (in the figure, formats 2 to n). Accordingly, each of the signals of these formats is converted into a signal of format 1 which display processing unit 103 can process, by a corresponding one among format converters 101a to 101n, and then supplied to display processing unit 103. Display processing unit 103 carries out processing such as drive processing of display unit 104 in response to the video signal of format 1 provided in this way.
In converting formats by each format converter, basically, it is necessary to convert the horizontal and vertical frequencies of an input video signal after considering the aspect ratio. Specifically, in format conversion, each format converter is required to carry out conversion processing of the number of scanning lines, companding processing of a video signal in the horizontal direction by considering the aspect ratio, conversion processing of the field frequency using a frame memory and so on, or the like.
In the case of the video display apparatus shown in FIG. 1, the display processing such as driving of display unit (LCD device) 104 by display processing unit 103 can be performed in a fixed manner regardless of the format of the input video signal.
However, there is a problem that a large-scale circuit structure is needed for the plurality of format converters. Furthermore, there is a problem that the timing of reading gets ahead of the timing of writing in the memory in converting a field frequency in each format converter using a finite image memory.
FIG. 2 is a block diagram showing one example of a video display apparatus for which a counter-measure different from that of the apparatus of FIG. 1 is taken. In the apparatus shown in FIG. 2, each of format converters 201a to 201n only performs conversion processing of the number of scanning lines and companding processing of a video signal in the horizontal direction considering the aspect ratio, and does not perform conversion processing of the vertical frequency, i.e., the field frequency as in the example of FIG. 1. Accordingly, the circuit scale of format converters 201a to 201n in FIG. 2 can be remarkably reduced compared with that of format converters 101a to 101n in FIG. 1.
In the video display apparatus shown in FIG. 2, however, conversion of the vertical frequency is not performed, so that the horizontal frequency of the video signal differs for each format. Accordingly, a display processing unit 203 of the apparatus of FIG. 2, unlike in the example of FIG. 1, cannot perform display processing of display unit (LCD device) 104 with one fixed format. For example, as the clock frequency employed in a driving processing system of display unit 104 differs with the format of the input video signal, display processing unit 203 must change its processing operation for each input video signal, and a problem results in that the circuit structure and the processing operation thereof are made complicated.